Three-dimensional multichannel waveguide grating filters

Distributed fiber gratings exhibit outstanding capabilities in achieving a wide spectral response through the superimposition of gratings with different periods in the fiber core. This significantly broadens the design flexibility and potential applications of fiber gratings. However, as photons pass through gratings with varying periods in sequence, which not only inevitably existing signal crosstalk but also poses challenges for integrating. In this study, a three-dimensional (3D) four-channel filter is proposed and realized in fiber-compatible materials using femtosecond laser writing. The filter consists of a 3D beam splitter and four parallel different-period Bragg waveguide gratings (WGs). By designing grating periods in each path, parallel filtering and reflection at multiple designed wavelengths are achieved compactly with 50 nm spectrum spacing within 1450–1600 nm wavelengths. The four-channel filter entire measures 15.5 mm × 1 mm × 1 mm (the highest integration of distributed fiber gratings reported so far). Our technique will augment the laser fabrication technology for 3D integrated photonic devices and serve as a powerful and generalized solution for highly integrated in-situ measurement and multi-parameter decoupled sensing.